Provide convenience methods for required objects

src/main/java/net/fortuna/ical4j/model/Calendar.java

@@ -225,7 +225,12 @@ public final <C extends CalendarComponent> ComponentList<C> getComponents(final
      * @return the first matching component in the component list with the specified name
      */
     public final <T extends CalendarComponent> Optional<T> getComponent(final String name) {
-        return (Optional<T>) getComponents().getComponent(name);
+        return getComponents().getComponent(name);
+    }
+
+    public final <T extends CalendarComponent> T getRequiredComponent(String name) throws ConstraintViolationException {
+        Optional<T> component = getComponent(name);
+        return component.orElseThrow(() -> new ConstraintViolationException(String.format("Missing %s component", name)));
     }
 
     /**
@@ -253,6 +258,11 @@ public final <T extends Property> Optional<T> getProperty(final String name) {
         return getProperties().getProperty(name);
     }
 
+    public final <T extends Property> T getRequiredProperty(String name) throws ConstraintViolationException {
+        Optional<T> property = getProperty(name);
+        return property.orElseThrow(() -> new ConstraintViolationException(String.format("Missing %s property", name)));
+    }
+
     /**
      * Perform validation on the calendar, its properties and its components in its current state.
      * @throws ValidationException where the calendar is not in a valid state

src/main/java/net/fortuna/ical4j/model/Component.java

@@ -198,9 +198,9 @@ public <T extends Property> Optional<T> getProperty(final String name) {
      * @return the first matching property in the property list with the specified name
      * @throws ConstraintViolationException when a property is not found
      */
-    protected final <T extends Property> T getRequiredProperty(String name) throws ConstraintViolationException {
-        return (T) getProperties().getProperty(name).orElseThrow(() ->
-                new ConstraintViolationException(String.format("Missing %s property", name)));
+    public final <T extends Property> T getRequiredProperty(String name) throws ConstraintViolationException {
+        Optional<T> property = getProperty(name);
+        return property.orElseThrow(() -> new ConstraintViolationException(String.format("Missing %s property", name)));
     }
 
     /**

src/main/java/net/fortuna/ical4j/model/ComponentList.java

@@ -95,10 +95,10 @@ public final String toString() {
      * @param aName name of component to return
      * @return a component or null if no matching component found
      */
-    public final Optional<T> getComponent(final String aName) {
+    public final <R extends T> Optional<R> getComponent(final String aName) {
         for (final T c : this) {
             if (c.getName().equals(aName)) {
-                return Optional.of(c);
+                return Optional.of((R) c);
             }
         }
         return Optional.empty();

src/test/java/net/fortuna/ical4j/model/property/ExDateTest.java

@@ -83,8 +83,8 @@ public void testTimeZones() throws Exception {
         CalendarBuilder builder = new CalendarBuilder();
         Calendar calendar = builder.build(getClass().getResourceAsStream("/samples/valid/EXDATE.ics"));
 
-        Optional<VEvent> event = calendar.getComponent(Component.VEVENT);
-        List<Property> exdates = event.get().getProperties(Property.EXDATE);
+        VEvent event = calendar.getRequiredComponent(Component.VEVENT);
+        List<Property> exdates = event.getProperties(Property.EXDATE);
         for (Property exDate : exdates) {
             assertTrue("This EXDATE should have a timezone", exDate.getParameter(Parameter.TZID).isPresent());
         }
@@ -94,25 +94,25 @@ public void testDstOnlyVTimeZones() throws Exception {
         CalendarBuilder builder = new CalendarBuilder();
 
         Calendar ical = builder.build(getClass().getResourceAsStream("/samples/valid/dst-only-vtimezone.ics"));
-        Optional<VTimeZone> vTZ = ical.getComponent(VTimeZone.VTIMEZONE);
+        VTimeZone vTZ = ical.getRequiredComponent(VTimeZone.VTIMEZONE);
 
-        String id = vTZ.get().getProperty(Property.TZID).get().getValue();
+        String id = vTZ.getRequiredProperty(Property.TZID).getValue();
         assertEquals("Europe/Berlin", id);
-        assertEquals(vTZ.get().getObservances().get(0), vTZ.get().getApplicableObservance(TemporalAdapter.parse("20180403").getTemporal()));
+        assertEquals(vTZ.getObservances().get(0), vTZ.getApplicableObservance(TemporalAdapter.parse("20180403").getTemporal()));
 
-        Optional<VEvent> vEvent = ical.getComponent(VEvent.VEVENT);
-        Optional<DtStart<?>> start = vEvent.get().getStartDate();
-        Optional<TzId> startTzId = start.get().getParameter(Parameter.TZID);
-        assertTrue(startTzId.equals(vTZ.get().getTimeZoneId().get().getParameter(Parameter.TZID)));
-        assertEquals(1522738800000L, Instant.from(start.get().getDate()).toEpochMilli());
+        VEvent vEvent = ical.getRequiredComponent(VEvent.VEVENT);
+        DtStart<?> start = vEvent.getRequiredProperty("DTSTART");
+        Optional<TzId> startTzId = start.getParameter(Parameter.TZID);
+        assertEquals(startTzId, vTZ.getRequiredProperty("TZID").getParameter(Parameter.TZID));
+        assertEquals(1522738800000L, Instant.from(start.getDate()).toEpochMilli());
     }
 
     public void testShouldPreserveUtcTimezoneForExDate() throws Exception {
         CalendarBuilder builder = new CalendarBuilder();
         Calendar calendar = builder.build(getClass().getResourceAsStream("/samples/valid/EXDATE-IN-UTC.ics"));
 
-        Optional<VEvent> event = calendar.getComponent(Component.VEVENT);
-        List<Property> exdates = event.get().getProperties(Property.EXDATE);
+        VEvent event = calendar.getRequiredComponent(Component.VEVENT);
+        List<Property> exdates = event.getProperties(Property.EXDATE);
         for (Property exDate : exdates) {
             for (Instant dateEx : ((ExDate<Instant>) exDate).getDates().getDates()) {
                 assertNotNull(dateEx);
@@ -125,7 +125,7 @@ public void testShouldPreserveUtcTimezoneForExDate() throws Exception {
      */
     public void testRelaxedParsing() throws DateTimeParseException {
         try {
-            new ExDate(new ArrayList<>(), "20080315");
+            new ExDate<Instant>(new ArrayList<>(), "20080315");
             fail("Should throw DateTimeParseException");
         } catch (DateTimeParseException pe) {
             LOG.trace("Caught exception: " + pe.getMessage());